logon script: clarify usage for different Samba roles

logon script: clarify usage for different Samba roles

'logon script' is only used when Samba runs as a logon server in the
classical domain controller role. And even there 'ldapsam' passdb
backend will override it.

For Samba AD setup 'logon script' is not used at all.

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>